This invention relates to heat strengthened glass and in particular to a method and apparatus for manufacturing heat strengthened glass panels for incorporation in a fully-glazed facade of a building.
It is desirable that such glass panels should be stronger than annealed glass so as to be capable of resisting stresses to which the panels may be subjected during glazing, and due to wind load when in situ. However the panels should not be heat treated to a degree such that thermally induced stresses in the glass are at a level which could cause dicing of the glass upon inadvertent fracture.
Glass sheets which satisfy this requirement as known as "heat strengthened glass" which is defined in U.S. Federal Spec. No. DD-G-14O3D, dated August 15, 1972 as glass having a surface compression of not less than 3,500 p.s.i. (24 MN/m.sup.2) or greater than 10,000 p.s.i. (69 MN/m.sup.2) or an edge compression of less than 5,500 p.s.i. (38 MN/m.sup.2).
The permissible upper limit of stress is dependent on the thickness of the glass and it has been found that the thicker the glass the lower is the value of the stresses in the glass which may cause dicing when the glass is fractured. Thus thicker heat strengthened glass, for example about 12 mm thick, may have compression stresses somewhat lower than those indicated above, while thinner heat strengthened glasses may have compression stresses somewhat higher than those indicated above.
It has been usual to produce heat strengthened glass sheets, for architectural use, on a roller furnace in which the glass is heated to a temperature above its strain point and is then cooled while it is horizontally supported. Usually this cooling is carried out by indexing the glass sheet to and fro on horizontal rollers while subjecting the glass surfaces to cooling flows, which are usually cooling air flows which are directed towards the upper and lower glass surfaces.
In order to allow flows of cooling air between the glass surface and the rollers, it has been usual to employ rollers which do not contact the lower surface of the glass along the whole length of the rollers.
Such rollers which have been employed are "donut" rollers having a spaced series of ceramic tires which provide the roller support, or rollers which are wound helically with thermally insulating tape of refractory fibre material. Both the tires and the helical windings provide bearing surfaces which are continuous across the region of contact between the roller and the glass.
The Applicant is aware of GB No. 1240502 and U.S. Pat. No. 2,130,282 (Drake), the latter specification corresponding to GB No. 472516. GB No. 1240502 describes a process in which glass is cooled by streams of gas issuing through horizontal rollers which provide a continuous supporting surface for the glass (although at high gas flow rates, the gas pressure may be sufficient to support the glass above the rollers). U.S. Pat. No. 2,140,282 describes a toughening furnace in which the glass is supported on horizontal rollers and then withdrawn over "donut" rollers of the kind referred to above; these "donut" rollers have support surfaces which are continuous around the periphery of the roller tires.
These specifications are primarily concerned with the production of strengthened glass. When the glass is cooled to toughen it, heat transfer takes place predominantly between the cooling medium (usually air) and the glass, and heat transfer between the glass and supporting rollers is not so significant.
When manufacturing heat strengthened thick glass, particularly in thickness of 6 mm or more, for example about 10 mm thick, the level of stress to be induced in the glass predicates a low rate of heat exchange with the glass surfaces during cooling. The thicker the glass, the lower the rate of cooling, and the heat transfer from the lower glass surface by conduction through the roller contact is then at a rate commensurate with heat transfer from the glass due to the overall cooling of exposed parts of the glass surface between the rollers. Thus the heat transfer between the glass and the supporting roller surfaces becomes significant and leads to patterns of iridescence in the glass. Patterns of iridescence induced in the glass, which are visible in polarised light, have been found to include prominent continuous band or trellis-like features which would be visible to the eye when the panels are glazed in a building. Such patterns are unacceptable, and are emphasised when the glass has been coated after its heat treatment, because the presence of a light reflecting coating on the glass can enhance the obtrusive nature of such iridescent patterns.
It is a main object of this invention to provide a solution to this problem which results in heat strengthened glass panels for architectural use in which any such patterns of iridescence are of a random or discontinuous nature, and are unobtrusive, being no worse than the kind of iridescent patterns which are usually observed in thermally toughened glass.